White liquor consists of a water solution of the active digestion chemicals sodium hydroxide ("NaOH") and sodium hydrogen sulfide ("NaHS"), which are included in the term "alkali". The alkali in the white liquor is consumed during the digestion of cellulosic fiber material by reacting with both lignin and carbohydrates, such as cellulose and hemicellulose. The higher the concentration of sodium hydrogen sulfide compared to the concentration of sodium hydroxide, the higher the sulfidity. The sulfidity represents the weight percentage of alkaline sulfur containing compounds present based on the total weight of alkaline compounds present. The sulfidity of the white liquor is dependent upon several different factors present in a sulfate factory of the pulp plant, and a general aim is to increase the sulfidity in the white liquor. The sulfidity can be increased by various measures, such as by closing the chemical system in the factory and recovering sulfur from various gas discharges. In this way the sulfidity level can be increased from an initial level of 30% to a level of about 45%.
The sulfidity is a significant process variable and therefore research is directed to further increasing the sulfidity. An increase in sulfur or sulfur containing compounds that are capable of producing hydrogen sulfide ions ("HS.sup.- ") in the digesting liquor results in a quicker digestion process, higher pulp yield and better pulp quality. This is because HS.sup.- ions are capable of reacting with lignin through different reaction paths than hydroxyl ions so that the delignification occurs at higher speed and thus chemical attack of the carbohydrates is reduced.
Published Swedish patent application No. SE-9202996-5 discloses adding recovered hydrogen sulfide to the impregnation of wood chips prior to delignification. However, such an addition is difficult to perform because hydrogen sulfide is an exceptionally toxic gas and it must be added to a pressurized chip bed. It is also difficult to dispose of or reuse excess hydrogen sulfide after impregnation.
U.S. Pat. No. 3,841,962 describes a method of pretreating chips with an alkali enhanced liquor. The alkali enhanced liquid is produced by crystallizing and separating Na.sub.2 CO.sub.3 out of a green liquor in order to obtain an increased concentration of alkali, in particular NaHS and NaOH. The alkali enhanced liquor is supplied to the pretreatment tank, and CO.sub.2 is added to effect carbonation and the formation of NaHCO.sub.3 and H.sub.2 S in situ. After the pretreatment, a liquid is withdrawn which contains remnants of NaHCO.sub.3 and dissolved H.sub.2 S and CO.sub.2. This method has the problem that it is difficult to dispose of this liquid. Furthermore, the pretreatment liquid must be made from freshly produced green liquor which requires treatment to separate the Na.sub.2 CO.sub.3 followed by addition of CO.sub.2. Moreover, this method limits the desired excess of H.sub.2 S at the pretreatment stage.
A limiting factor for increasing the sulfidity in white liquor is mainly the use of a soda boiler for recovering digestion chemicals and the necessary causticizing of the green liquor produced from melt from the soda boiler.
Published Swedish patent application No. SE-B-468 600 discloses a method for obtaining white liquor of high sulfidity directly from the gasification reactor, without the need for causticizing. According to this procedure, hydrogen sulfide is recovered from gas leaving a reactor for the thermal decomposition of black liquor and the hydrogen sulfide is returned to the reactor. Such a high partial pressure of hydrogen sulfide is thus established in the gasification stage that the equilibrium reaction: EQU Na.sub.2 CO.sub.3 +H.sub.2 S.revreaction.Na.sub.2 S+CO.sub.2 +H.sub.2 O
is displaced so far to the right that the formation of Na.sub.2 CO.sub.3 is suppressed. The Na.sub.2 S formed is easily dissociated in water to NaOH and NaHS. The recovery of hydrogen sulfide from the gas step is accomplished by passing the gas through a gas scrubber containing an external absorption chemical, such as N-methyl pyrolidone or methyl di-ethyl amine, for selective and regenerative absorption of H.sub.2 S. This process has two significant drawbacks, (1) the need for external chemicals and (2) the requirement of a regeneration step to strip the hydrogen sulfide from the absorption chemical.
There is a need for a process of making pulp which increases the sulfidity in the digester and avoids the above problems associated with conventional pulp production processes.